Reinforced ceramic matrix composites ("CMC's") are well suited for structural applications because of their toughness, thermal resistance, high temperature strength and chemical stability. These composites can be produced by adding whiskers, fibers or platelets to a ceramic matrix. In the fabrication of continuous fiber reinforced-ceramic matrix composites ("CFCC's"), the fabrication process usually begins by weaving continuous TM fiber tows (e.g., sintered SiC fibers such as Hi-Nicalon or Dow Corning Sylramic.TM.) into a cloth such as 2-dimension 5HS or 8HS, or 3-dimension cloths. The woven fiber cloth is then formed into a panel or shape called a fiber preform. The porosity within the fiber preform is then filled to produce the dense CFCC. The non-brittle nature of the CFCC provides the much needed reliability that is otherwise lacking in monolithic ceramics.
The enhanced fracture resistance of ceramic matrix composites is achieved through crack deflection, load transfer, and fiber pull-out. Fiber pullout is achieved by having little or no chemical bonding between the fibers and matrix, so that the fibers are able to slide along the matrix. However, it is also known that many fiber-matrix combinations undergo extensive chemical reaction or interdiffusion between the fiber and matrix materials during densification. Such reaction or interdiffusion can lead to serious degradation in strength, toughness, temperature stability and oxidation resistance. Accordingly, the proper fiber-matrix interface must be selected in order to prevent or minimize chemical reactions and interdiffusion.
Surface modification of the fibers is an effective means to control reaction at the fiber-matrix interface. This can be accomplished by coating the fibers with a suitable ceramic. Equally important, a suitable ceramic coating also allows the debonding of the fiber's matrix interface and enables the fiber to pull out from the matrix and slide along the matrix, thus increasing the fracture toughness of the composite. Coated silicon carbide fibers and whiskers are known in. The art of composite materials. U.S. Pat. No. 4,929,472 ("Sugihara") discloses SiC whiskers having a surface coated with either a carbonaceous layer or a silicon nitride layer. These surface coated whiskers are used as a reinforcing material for ceramics such as SiC, TiC, Si.sub.3 N.sub.4, or Al.sub.2 O.sub.3. U.S. Pat. No. 4,781,993 to Bhatt discloses a SiC fiber reinforced reaction bonded Si.sub.3 N.sub.4 matrix wherein the SiC fibers are coated with an amorphous carbon layer and an overlayer having a high silicon/carbon ratio covering the amorphous layer. U.S. Pat. No. 4,642,271 to Rice discloses BN coated ceramic fibers embedded in a ceramic matrix. The fibers may be SiC, Al.sub.2 O.sub.3 or graphite, while the matrix may be SiO.sub.2, SiC, ZrO.sub.2, ZrO.sub.2 -TiO.sub.2, cordierite, mullite, or coated carbon matrices. U.S. Pat. No. 4,944,904 to Singh et al. discloses a composite containing boron nitride coated fibrous material. Carbon or SiC fibers are coated with BN and a silicon-wettable material and then admixed with an infiltration-promoting material. This mixture is formed into a preform which is then infiltrated with a molten solution of boron and silicon to produce the composite.
The densification of green CFCC's is more difficult than that of green monolithic ceramics. Conventional sintering of a green ceramic matrix reinforced with sintered fibers is not possible, as the green ceramic matrix has rigid inclusions. Densification of green CFCC's can, however, be achieved by chemical vapor infiltration ("CVI") or molten silicon infiltration. Molten silicon infiltration is the preferred method because it is less time consuming and more often produces a fully dense body than the CVI process. For high temperature applications, full densification is necessary for good thermal and mechanical properties and for preventing rapid oxidation/degradation of the reinforcements or reinforcement coating. For example, desirable characteristics for CFCC's used in air transport applications include a high thermal conductivity, high tensile strength, high tensile strain and a high cyclic fatigue peak stress. One conventional CFCC fabricated by state-of-the-art chemical vapor infiltration processing has been found to have a thermal conductivity of only about 4.7 BTU/hr.ft.F at 2200.degree. F., and a cyclic fatigue peak stress of only about 15 ksi (about 105 MPa) using a Hi-Nicalon.TM. fiber. It is believed the low thermal conductivity and cyclic fatigue peak stress of this CVI material is due to the material's relatively high porosity (typically 10-20%) which is common for CVI processes. According, the art has focused upon densification by silicon infiltration.
Densification by silicon infiltration has been practiced for monolithic ceramics, such as reaction-bonded silicon carbide, for many years. This process, as described in U.S. Pat. No. 3,205,043 to Taylor, involves infiltrating molten silicon through the pores of a green body containing alpha silicon carbide and carbon. The silicon reacts with the carbon to form beta-SiC, which then bonds the alpha-SiC grains together. The portion of the infiltrated molten silicon which does not react with the carbon solidifies upon cooling, thereby filling the pores of the Sic bonded Sic body. This phenomenon is known as siliconization, and results in a fully dense end product containing SiC and residual free silicon. Since silicon infiltration does not involve shrinkage of the green body (as is the case with conventional sintering), the final dense product is near net shape. The art has used silicon infiltration to densify fiber-containing ceramic composites as well.
U.S. Pat. No. 5,296,311 ("McMurtry"), the specification of which is incorporated by reference, discloses a silicon infiltrated silicon carbide composite reinforced with coated silicon carbide fibers. McMurtry discloses a process including the steps of:
a) coating SiC fibers with a coating selected from the group consisting of aluminum nitride, boron nitride and titanium diboride; PA1 b) treating the surface of the coated fibers with a mixture of SiC powder, water and a non-ionic surfactant; PA1 c) preparing a slurry comprising SiC powder and water; PA1 d) impregnating the coated fibers with the slurry using a vacuum dewatering process to form a cast; PA1 e) drying the cast to form a green body; and PA1 f) silicon infiltrating the green body to form a dense SiC fiber reinforced reaction bonded matrix composite. PA1 a) providing a fiber preform comprising a non-oxide ceramic fiber having at least one coating, the fiber and coating each optionally having a degradation temperature of between 1410.degree. C. and 1450.degree. C., the coating comprising an element selected from the group consisting of carbon, nitrogen, aluminum and titanium, PA1 b) impregnating the preform in a porous mold with a slurry comprising silicon carbide particles and between 0.1 and 3 wt % added carbon to produce an impregnated green body, PA1 c) providing a cover mix comprising: PA1 d) placing the cover mix on at least a portion of the surface of the impregnated green body, PA1 e) heating the cover mix to a temperature between 1400.degree. C. and 1500.degree. C. to melt the alloy (optionally, between 1410.degree. C. and 1450.degree. C.), and PA1 f) infiltrating the green body with the melted alloy for a time period of between 15 minutes and 240 minutes, to produce a ceramic fiber reinforced ceramic composite.
McMurtry reports that providing the disclosed coatings on SiC fibers limited both mechanical and chemical bonding with the matrix, and so improved the strength and toughness of the composite material. However, CFCC's produced in substantial accordance with McMurtry have been found to have a four point flexure strength at room temperature of only about 1 ksi. Since the tensile strength of a ceramic is typically only about 60%-90% of its four point flexure strength, these CFCC's likely have a tensile strength of only about 0.6-0.9 ksi. Further assuming an elastic modulus of about 30 million psi, these CFCC's likely have an ultimate tensile strain of less than 0.003% at room temperature. The reason for these low values is believed to be the low strength of the fiber used in McMurtry, as well as the partial reaction of the debonding coating with the molten silicon. Moreover, simple substitution of higher strength SiC fibers, such as Hi-Nicalon fiber, presents more severe degradation problems because the these higher strength fibers are considered to be more susceptible to degradation by molten silicon than the SiC fibers used by McMurtry. In particular, these higher strength fibers typically degrade in the temperature range of only about 1410-1500.degree. C. while the silicon infiltration step in McMurtry is undertaken at a temperature of about 1500.degree. C.
In addition, one specific problem encountered with SiC reinforced SiC composites fabricated by a silicon infiltration process is that the SiC fiber or coating thereon may react with the molten silicon during infiltration, resulting in the degradation of the composite's desirable properties. For example, it has been found that, due to the high reactivity of molten silicon, the BN debonding coating is also attacked during the silicon infiltration step, resulting in severe degradation of the underlying SiC fiber and hence the CFCC properties. To reduce such attack, a duplex coating concept in which a second "protective" coating of CVD-SiC is deposited on top of the BN coating has been studied. See, e.g., U.S. Pat. No. 4,944,904. While the CVD-SiC coating is more stable than the underlying BN coating in the presence of molten silicon, it has been found that molten silicon still dissolves the CVD SiC coating considerably. As a result, the silicon melt infiltration process has to be conducted at a relatively low temperature (i.e., close to the melting point of silicon, which is 1410.degree. C.) and for a short time (less than 30 minutes). Because of this abbreviated infiltration step, the resulting CFCC microstructures often have incomplete silicon infiltration, high porosity and poor thermo-mechanical properties. A second aspect of the conventional process as typified by U.S. Pat. No. 4,889,686 which limits the completeness of silicon infiltration is the use of carbon in the impregnation slurry. During the slurry impregnation step, the coated fiber tows or fabrics are impregnated with carbon, which is typically present as at least 10 wt % of the slurry. The infiltrated fiber tows or fabrics are then placed in a vacuum furnace and heated in the presence of molten silicon. The infiltrated carbon quickly reacts with molten silicon to form a beta SiC matrix. According to McMurtry, the presence of carbon in the slurry provides a reactant for forming the matrix SiC, and is believed to improve the wetting behavior of molten silicon and so allows the silicon to penetrate deeper into the fiber tow interior. The beneficial effects of the impregnated carbon during silicon infiltration is widely accepted. For example, the General Electric Toughened Silcomp.TM. process uses a slurry with at least 10 wt % carbon. However, since the reaction between silicon and carbon is a highly exothermic one, the heat generated by this reaction can cause severe localized heating of the fiber preform to between 100 and 200 degrees C above the intended molten silicon temperature. Since the stability of the higher strength Sic fibers and some debonding coatings (such as BN) are very sensitive to temperature, degradation of the coatings and the fibers are frequently encountered. One approach for decreasing this degradation is to limit the time and temperature at which silicon infiltration is performed. As a result of the relatively low temperatures used in the silicon infiltration step, siliconization is often incomplete and unreacted carbon remains. Moreover, it has been observed in conventional processing that the silicon/carbon reaction near the surface regions of the green CFCC often blocks the subsequent flow of silicon into the green CFCC interior, causing localized porous areas; its volume change causes cracking in the near net-shape components; and unreacted free carbon in the composite degrades its high temperature oxidation resistance.
In a third aspect of the conventional process, silicon infiltration is carried out by placing several large chunks of solid silicon at various locations on top of the impregnated green material and heating the silicon to its melting point. In theory, the infiltration process relies primarily on the capillary action of the liquid silicon or the gaseous transport of silicon vapors to permeate the porous green CFCC preform and to react with the impregnated carbon in the preform to form in-situ SiC. Although this process works well for monolithic ceramics, wherein infiltration is usually conducted at relatively high temperatures (at least 1750.degree. C.) which make the infiltration kinetics very fast, it does not work well with fiber preforms. Due to the limited thermal stability of the higher strength fibers and interface coating system, the temperature in CFCC's during molten silicon infiltration has to be kept very close to the melting point of silicon (about 1410.degree. C.). Since the infiltration kinetics are very slow at these lower temperatures, it takes an exceedingly long time for the molten silicon to wick or spread to areas not directly under a silicon chunk. This results in either nonuniform infiltration characterized by many porous areas or severe fiber/coating attack if the infiltration process is allowed to proceed for a much longer time to complete the infiltration. In either case, an inferior CFCC is produced. Secondly, with this technique it is also extremely difficult to control the net amount of silicon infiltrated into the fiber preform. As a result, extra silicon in the form of surface lumps is usually observed on the CFCC exterior. Although post-infiltration machining of these lumps can be undertaken, it not only increases the fabrication cost of the CFCC, it also degrades its CFCC properties.
Therefore, conventional CFCC's made by silicon infiltration processes typically contain fibers which are either heat resistant at typical silicon infiltration temperatures but have low strength, or contain fibers which have high strength but are susceptible to degradation at typical silicon infiltration temperatures. Conventional CFCC's made by CVI processes typically have high porosity, and so have low thermal conductivity, low cyclic fatigue peak stress at high temperatures, and low resistance to oxidation.
Accordingly, there is a need for a CFCC having a high thermal conductivity, a high cyclic fatigue peak stress at high temperatures, a high ultimate tensile strain, and a high ultimate tensile strength.
In a fourth aspect of the conventional process, it has been observed that the surface texture of the composite after silicon infiltration has the same highly rough woven structure of the fiber preform. For applications such as turbine or aerospace components that need an aerodynamic surface finish, such a rough surface can result in reduced performance. One proposed solution to the surface roughness problem is to deposit a layer of CVD SiC on the impregnated preform surface and then machine it to the desired surface finish. The disadvantage of this approach is that it is difficult and costly to machine the hard CVD SiC coating.